Children with sickle cell disease (SCD) may suffer devastating complications. However, fetal hemoglobin levels in children with SCD are generally higher than those in adults. The response of children to Hydroxyurea (HU) therapy may be quite different from that of adults. Small scale Phase II studies have been conducted in a few institutions, but a larger scale closely monitored Phase II study in children is needed as a preliminary study to efficacy studies. In this study, we have organized a consortium of pediatric clinical investigators to conduct an open label phase II multi-institutional trial of HU in children aged 5 to 15 years with HB SS disease. Our goal is to obtain preliminary data on the following questions: (1) Is hydroxyurea effective in increasing fetal hemoglobin levels in children?, (2) Are there serious toxicities in children that were not seen in adults?, and (3) what is the impact of hydroxyurea therapy on growth?